1. Field of the Invention
The present invention relates to a vacuum interrupter having a highly evacuated vessel containing a pair of electrode contacts which are in contact with each other when the vacuum interrupter is closed and with one of the electrode contacts being separated from the other electrode contact when the vacuum interrupter is open, an arc shielding member surrounding both the electrode contacts so as to prevent generation of metal vapor due to arcing between the electrode contacts during the opening and closing operation, and a cylindrical insulating envelope with metallic end plates located at the ends of the insulating envelope in which the pair of electrode contacts and the arc shielding member are contained in a highly evacuated state.
2. Statement of the Prior Art
A conventional vacuum interrupter comprises:
(1) an insulating envelope having a metallized portion covered by a metal suitable for hermetically brazing each of upper and lower ends thereof; PA1 (2) a first metallic end plate brazed to the upper end of the insulating envelope; PA1 (3) a second metallic end plate brazed to the lower end of the insulating envelope; PA1 (4) a stationary electrode holder extending through the center of the first metallic end plate and brazed thereat to the first metallic end plate; PA1 (5) a movable electrode holder extending through the center of the second metallic end plate so as to move vertically with one end thereof brazed to one end of a bellows whose other end is brazed to the center of the second metallic end plate; PA1 (6) a stationary electrode contact brazed to the inner end of the stationary electrode holder; PA1 (7) a movable electrode contact brazed to the inner end of the movable electrode holder; and PA1 (8) a cylindrical arc shielding member extended so as to surround both the stationary and movable electrode contacts, one end thereof being brazed to the second metallic end plate.
(the insulating envelope, first metallic end plate, and second metallic end plate constituting a vacuum vessel)
In such a conventional vacuum interrupter, the insulating envelope is formed of ceramics or crystallized glass (which is also called Devitro ceramics, glass ceramics, or devitrified glass). Such a material has a higher mechanical strength and superior heat resistance of 600.degree. C. or more. The end plates to be brazed to the insulating envelope have a thermal expansion coefficient different from that of the insulating envelope. After brazing a residual stress is, therefore, generated so that the insulating envelope may be destroyed since its mechanical strength is weaker than that of the end plates.
To cope with the problem described above, each end plate is formed of an alloy of iron and nickel (abbreviated Fe-Ni alloy) or iron, nickel, and cobalt (abbreviated Fe-Ni-Co alloy). Such alloys have substantially the same thermal expansion coefficient as ceramic or crystallized glass. A brazing alloy of 72% of silver and 28% of copper equal in solidus and liquidus temperature is frequently used as the brazing agent. If a brazing alloy including a component of silver is used, the brazing alloy penetrates and diffuses into the Fe-Ni alloy or Fe-Ni-Co alloy of the end plates to cause cracking, so that the reliability of the hermetic seal is lowered in the vacuum interrupter. Consequently, brazing alloys including silver cannot be used. Fe-Ni and Fe-Ni-Co alloys are magnetic materials and each end plate is positioned perpendicularly with respect to each electrode holder through which a current passes so that the magnetic flux passes perpendicularly through each end plate and the amount of induced flux is increased and a large eddy current is developed in each end plate. Consequently, each end plate exhibits a considerable temperature rise due to eddy current losses. This is particularly noticeable in the case of a vacuum interrupter with a large current rating. Furthermore, the use of cobalt is expensive and therefore an Fe-Ni-Co alloy is also expensive. As compared with the Fe-Ni-Co alloy, the Fe-Ni alloy is inexpensive but the difference of thermal expansion coefficient from that of ceramics or crystallized glass is relatively large and residual stresses will be easily generated.